This invention relates to a wedge for bonding wire to a bond pad. In particular, it relates to a wedge having an aperture through which a wire passes, sidewalls that constrain the sideways movement of the wire, and a dropped foot that presses the wire against a bond pad and bonds it to that bond pad.
A wedge is a tool for bonding wire to a bond pad, which is typically on a printed circuit board that will become part of a micro-electronic device. The bond may be made to a very small bond pad, recessed about 2 to 8 microns below a protective overcoat or passivation layer. The wire passes through an aperture in the wedge, then over a foot that is pressed against the bond pad. The foot bonds the wire to the bond pad, typically by means of ultrasonic vibrations. After a first bond has been made, the wedge is moved to a second bond pad, where a second bond is made, and then the wire is cut off. The first bond and second bond may not be in-line with one another, but at an angle or diagonally spaced.
Two types of wedges are now in use commercially, the pocket type and the open notch type. FIG. 1 illustrates a prior art open notch type of wedge 1. Wire 2 passes through aperture 3 of wedge 1 into an open V-shaped notch 4, then over foot 5, which bonds it to a bond pad (not shown). When wire 2 passes through aperture 3 into open v-shaped notch 4, the absence of any side wall containment permits wire 2 to stray from side to side. This may happen due to wire memory or to the required bonding motions made by the wire bonding machine as it moves to different bonding positions. An off-center wire may lead to bonds that are off-center on the bond pad, or the wire may come out from under the foot of the wedge entirely so that no bond is made; if the wire is not beneath the foot, the foot may touch the bond pad and damage it when it attempts to form a bond.
FIG. 2 illustrates a prior art pocket type wedge 6. Wedge 6 has an aperture 7 through which wire 8 passes into pocket 9 formed by sidewalls 10. Wire 8 then moves under foot 11 which presses it against a bond pad (not shown) and forms a bond. Sidewalls 10 prevent wire 8 from moving side-to-side so that the wire is more centered on the foot than it is with the open notch wedge shown in FIG. 1 and the bond is more precisely positioned. However, the sidewalls of the pocket type may contact and damage surfaces on the microelectronic device, especially if the bond pad is slightly recessed, a problem that is avoided by the open notch design.
In FIG. 3, the pocket type of wedge of FIG. 2 is shown bonding wire 8 to recessed bond pad 12 of a microelectronic device 13. Because bond pad 12 is recessed, sidewalls 10 touch protective overcoat or passivation layer 14, which surrounds bond pad 12, causing cracking or other damage to overcoat or passivation layer 14. Additionally, in the current pocket type design, the back radius 15 of foot 11 cannot extend across the entire width of the wedge (see FIG. 2), causing a compromised radius geometry and an imperfect radius impression in the bonded wire, as well as difficulty in physically measuring back radius 15 on wedge 6 without destructive cross-sectioning.
Unlike the prior art pocket type of wedge, the prior art open notch type of wedge bonds wire to a recessed pad area without damaging the surrounding protective overcoat or passivation. The open notch design also has a uniform radius at the back of the foot across the entire width of the wedge, thereby making a uniform impression in the bonded wire and permitting the back radius to be measured easily and accurately. A uniform impression in the wire is beneficial for consistent wire bonding performance. However, as hereinabove pointed out, the prior art open notch type of wedge does not center the wire over the bond pad as well as the pocket type of wedge.